Cascode switches are typically designed with two or more MOSFETs (metal oxide semiconductor field effect transistors) or IGBTs (insulated gate bipolar transistors) connected in series. For example in a two transistor cascode switch, one transistor is coupled to the load and the second transistor is coupled in series between the first transistor and ground. The transistors are switched on and off in order to switch the load current as demanded or required. The load voltage is distributed across all of the series connected power transistors included in the cascode switch. For example, two 800V rated MOSFETs may be connected in series for switching a 1000V or greater load.
A prevalent issue with cascode switches is how to drive the high-side MOSFET or IGBT i.e. the transistor coupled closest to the load. In conventional approaches, a high voltage source such as a Zener diode in series with a resistor is used to turn on the high-side transistor, and the load current flowing through the high-side transistor is used to passively turn off the transistor. The low-side transistor i.e. the transistor coupled closest to ground is switched on and off with a pulsed low voltage source. The disadvantage of this approach is that the passive turn off of the high-side transistor is highly dependent on load current, and during turn off of the high-side transistor there is high dissipation on the low-side transistor voltage limiter.